Fuzz the fast renderer against the default one, and fix the KeyError it finds

[maartenbreddels]

Why

The fast renderer (REACTON_FAST=1) is tested by running the test suite with REACTON_FAST=1. Those tests were written for the default renderer, so they rarely hit the cases the fast renderer skips: an element that is the same object in the next render, in a subtree that did not change. Bugs in those skips went unnoticed.

What

Two commits:

1. The fuzz test (reacton/fuzz_test.py, from perf: make fast-renderer updates 10-60x cheaper and fix a state memory leak #58). It renders random component trees that change shape with their state (container type flips, keys, shuffles, fragments, wrappers, effects that set state, caught exceptions) and sets random state. After every step it checks that both renderers give the same widgets and run the same effects in the same order. On master, seed 9 of the 12 fails with a KeyError in the fast renderer.
2. The fix.
   • The bug: a child element that is the same object in the next render keeps its previous subtree. But when the widget that holds it is replaced by a widget of another type (VBox → HBox), reconciliation first removes the old subtree, including the child's component contexts. So the fast renderer crashes with a KeyError. The default renderer does not.
   • The fix: while the render phase walks the children of a widget that replaces a widget of another type, the fast path is off, so the child is rendered again. This is about 15 lines, taken from perf: make fast-renderer updates 10-60x cheaper and fix a state memory leak #58 without its "skip equal children" speed-up.

A child element is the same object in two renders when it comes from outside (a prop), or when it is the root element of a component that did not render again.

Tests

• Fuzz test, 300 seeds with 25 steps each (a local run, the PR keeps 12): 62 fail on master, all with this KeyError. With the fix, all 300 pass.
• test_replace_parent_same_child_element: the small case, which fails on master in the fast renderer.
• pytest reacton/ with REACTON_FAST=0 and =1: all pass.
• Solara's unit suite: the same results as master, in both renderers.

Review

Crossreview by three reviewers (astra, opus, glm): all three approve the fix.

• Checked and correct:
  • The guard uses the same key and condition as reconciliation.
  • The counter is restored in finally.
  • Renders, effects and cleanups are the same as in the default renderer, including nested replacements, a component whose root changes type, a .shared() child, and a replacement in a second render pass.
• Small fix, from the review (confirmed by the driver): the fuzz test now also checks for leaked widgets and callbacks (the cleanup_guard fixture only ran for core_test.py), and it compares whether widgets are closed.
• Not fixed here, an older bug: a child with an explicit .key() that moves out of a container whose type changes, into a sibling container, in the same render, still gives this KeyError in the fast renderer. It fails the same way on master. The default renderer handles it. The fuzz test cannot find it, because its children never move between containers. This is a follow-up.

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